The present invention is directed to a strain gage bridge circuit with sensitivity equalization and method for sensitivity equalization that are improvements over an invention disclosed in U.S. Pat. No. 4,979,580, issued Dec. 25, 1990 to Lockery, the contents of which patent are incorporated herein by reference.
U.S. Pat. No. 4,979,580 to Lockery discloses how to equalize the measuring sensitivity of opposing half-bridges in a strain gage bridge circuit by connecting pairs of shunt resistors of equal resistance cross a pair of bridge arms forming one half-bridge, and describes how this equalization method can be used to eliminate the sensitivity to variations in the location of the point of force application in a planar weighing device with four strain gages. Changes in the shunt resistance affects the sensitivity of one side of the load platform only, and does not affect the zero adjustment of the bridge output signal. The sensitivity adjustment method described in U.S. Pat. No. 4,979,580 to Lockery works well, and it has been used extensively. Sensitivity adjustment is usually done by trial and error, with resistor decade boxes used as shunt resistors. The resistor boxes are replaced with soldered-in combinations of fixed resistors before the load cell is shipped. Accurate replacement of the resistor box values, however, is a rather demanding and time consuming procedure, especially since the two resistors in a pair must always remain exactly equal. The shunt resistor values also affect the adjustments for linearity and temperature compensation in the strain gage bridge circuit.